An induction motor has played an important role in industry. Nowadays, an induction motor has been widely employed in many industrial applications, such as a fan or a pump. FIG. 1 shows a block diagram of an induction motor and the variable frequency drive thereof according to the prior art. As shown in FIG. 1, the induction motor 100 is driven by a variable frequency drive 101. An input AC voltage Vin is supplied to the input side of a rectifier 104 and is converted into a DC voltage Vdc by the rectifier 104. A capacitor Cdc 105 is located at the output end of the rectifier 104 for stabilizing the DC voltage Vdc and reducing the ripple of the DC voltage Vdc. The DC voltage Vdc is converted into an output AC voltage Vout and an AC output current Iout by an inverter 106, and the output AC voltage Vout and an AC output current Iout are supplied to the induction motor 100. The induction motor 100 is driven by the output AC voltage Vout and an AC output current Iout provided by the inverter 106 to power the load 102. Also, for the sake of controlling the output voltage and output current of the inverter 106, a voltage modulating module 108 is connected to the control terminals of the switching units Q1-Q4 in the inverter 106. The voltage modulating module 108 is configured to receive a voltage signal V and a frequency signal f to generate pulse signals PWM1-PWM4 that are used to control the operation of the switching units Q1-Q4 in the inverter 106. The switching units Q1-Q4 in the inverter 106 are driven to carry out switching according to the pulse signals PWM1-PWM4. A control unit 110 is connected to the voltage modulating module 108 for receiving the DC voltage Vdc, the output current Iout from the inverter 106, a target rotation frequency command signal f*1 given by the user via the user interface 112a, and a rotation frequency command signal f*2 given by the system via the system control circuit 112b. The control unit 110 is able to calculate the voltage signal V and the frequency signal f according to the rotation frequency command signal either f*1 or f*2, the DC-bus voltage Vdc and the output current Iout, for driving the voltage modulating module 108. Thus, the voltage modulating module 108 can control the inverter 106 to output the AC current Iout and the AC voltage Vout.
Generally, rotation speed control for an induction motor 100 is achieved by a variable frequency drive 101. However, when the variable frequency drive 101 starts the induction motor 100, the induction motor 100 may be still rotating freely due to the previous blackout or environment such as water flow, air flow, etc. If the induction motor 100 is driven forcedly under unknown free rotation speed, the induction motor will be easily damaged by frequent over-current condition or energy regeneration.
Currently, two solutions have been presented to tackle the problem that the induction motor is started under unknown free rotation speed. The first solution is called DC braking method. The DC braking method is featured by applying the DC voltage generated by the variable frequency drive 101 to the inductor motor 100, and the rotor of the induction motor 100 can be fixed according to electromagnetism. Nevertheless, the DC braking method has the following disadvantages. First, if the output voltage of the inverter 106 is so small that the magnetic force of the stator of the inductor motor 100 is too weak, the rotor is not easy to be fixed. If the output voltage Vout of the inverter 106 is too high, over-current conditions may happen. Second, the output duration of the inverter 106 can not be settled. If the output duration of the inverter 106 is too short, the rotation speed of the induction motor 100 can not be decelerated to fix the rotor. If the output duration of the inverter 106 is too long, the induction motor 100 will be overheated.
The second solution is called mechanical braking method. The mechanical braking method is featured by locking the rotor of the induction motor 100 with an additional latch. Therefore, the rotor shaft is kept stationary before the variable frequency drive 101 outputs the voltage. Nevertheless, the mechanical braking method has the following disadvantages. First, in order to lock the rotor of the induction motor 100, an extra mechanical latch is needed. This would increase the cost of the induction motor 100. Second, as the rotor shaft is susceptible to the load 102, the additional mechanical latch has to bear the stress of the load 102. However, the stress of the load 102 is an unknown variable. If the design of the mechanical latch is defective, the whole system may be damaged.
Therefore, a rotation speed searching apparatus for an induction motor is proposed in order to address the problems encountered by the prior art.